Manufacturing operations for the production of most polymers generally involve two main steps. The first step involves polymerization of the required starting materials to form the desired polymer. This step may be carried out in either a homogeneous system, that is bulk or solution polymerization, or in a heterogeneous system, that is suspension or emulsion polymerization. The second step in the manufacturing operation involves processing of the formed polymer for further shaping operations such as sheet formation, molding or extrusion. Reactive processing combines these two separate steps of polymer manufacture into a single step involving both polymerization and shaping of a premixed reactive precursor mixture. In this reactive processing procedure, the reactive mixture polymerizes and forms the desired product without further curing or shaping steps.
For example, to eliminate high drying costs and problems with solvent pollution, the coating industry has begun producing coatings by employing a mixture of monomer and polymer with subsequent heat or radiation polymerization of the monomer. However, these thermal and radiation steps are still expensive and add considerably to the physical size of the continuous coating operation. A viable alternative to thermal and radiation initiated polymerization involves mixing reactive precursor systems. In these reactive systems comprising monomers, oligomers, or mixtures thereof, polymerization commences when the individual components are brought into contact during mixing just prior to the point of coating. This eliminates the need for subsequent curing steps.
While premixing these activated systems can eliminate or reduce the need for high energy consuming curing steps and reduce the physical size of apparatus necessary for this coating process, premixed reactive precursor mixtures tend to build viscosity rapidly as they polymerize. The presence of solid die surfaces also causes shear flow leading to varying extents of reaction of these precursor mixtures and high viscosity gradients across any die cross section. When using premixed reactive precursor mixtures, these factors contribute to the formation of a layer of gelled polymer on the die walls. This gel layer forms a new surface boundary for the flow of the reactive precursor mixture on which more gel will continue to build upon. Ultimately, this continuously building gelled layer results in die plugging. Symptoms indicating this plugging phenomenon include a progressive increase in the pressure needed to drive the reactive precursor mixture flow at a constant flow rate through the die, a dimensionally reduced extruded product and the necessity to stop the extrusion process periodically for die cleaning.
Although a wide variety of extrusion dies are currently available, they uniformly fail to alleviate the specific problems mentioned above associated with the extrusion of polymers from premixed reactive precursor mixtures. Available articles illustrate this plugging problem. For example, reference can be had to: "Laminar Flow of a Thermosetting Polymer Through a Coat Hanger Die", 4 Polymer Process Engineering. No. 2-4, pp 151-171 (1986); "Laminar Tube Flow with a Thermosetting Polymerization", 28 AICHE Journal No. 6, pp 973-980 (Nov. 1982).